Police officers escort bystanders away as they secure the area after at least one person opens fire at a social services agency in San Bernardino, California, Dec 2, 2015. [Photo/Agencies]

Burguan said he knew of no possible motive behind the attack, which unfolded at about 11 a.m.

"We have no information at this point that this is terrorist-related, in the traditional sense that people may be thinking," he added. "Obviously, at minimum, we have a domestic-type terrorist-type situation that occurred here."

He said the suspects were armed with rifles.

The shooting unfolded on the campus of the Inland Regional Center, one of 21 facilities set up by the state and run under contract by non-profit organizations to serve people with developmental disabilities, said Nancy Lungren, spokeswoman for the California Department of Developmental Services.

Lavinia Johnson, executive director of the facility, told CNN the suspects opened fire inside a conference building in the complex where a holiday party was being held for county health department personnel.

The conference building sits adjacent to the two larger three-story buildings that house most of the agency's offices at the complex, Johnson said. Asked whether that meant that the Inland Regional Center staff and clients were safe, she said she understood they were being evacuated.